Airframe Aerodynamics of the CRW Demonstrator Aircraft in Hover with Winds

The Canard Rotor Wing demonstrator aircraft, X-50 Dragonfly, was tested in the Boeing V/STOL Wind Tunnel (BVWT) in late 2004 to investigate rotor/airframe aerodynamic interactions in hover and very low speed flight, in order to identify aeromechanical contributors to the accident of another X-50 aircraft in hover flight early in 2004, obtain aeromechanical data to improve X-50 flight simulation modeling, and validate CFD modeling of the X-50 aerodynamic characteristics. The CRW aircraft concept uses a tip-jet reaction-drive rotor in vertical and low speed forward flight modes. The rotor is stopped for high speed forward flight. The CRW concept uses exhaust gas and bypass air from a turbofan engine to power the rotor in helicopter mode. The wind tunnel testing was conducted with the BVWT test section in the open jet configuration. A six-component force and moment balance was used to acquire total aircraft force and moment data. The wind tunnel testing included wheel heights above the ground plane from five to thirty feet, and tunnel test section airspeeds of zero to ten knots. The aircraft was tested at several yaw angles to simulate hover conditions in the presence of winds from different directions. This paper focuses principally on the measured airframe aerodynamic pitching moment characteristics of the X-50 aircraft, although all six force and moment components obtained from the balance were used to improve X-50 flight simulation modeling. Wind direction was found to be more important to X-50 pitching moment than previously realized. Also, wind direction was found to be more important to X-50 pitching moment than wind speed, horizontal tail incidence, or wheel height. For horizontal tail at zero incidence, with aircraft pitch attitude at -10 degrees (nose-down), with 10 knots wind from the aft direction, the aircraft pitching moment (about the aircraft reference center) is about 370 foot-pounds nose-down. A CFD code, Rot3DC, was successfully used to predict the complex rotor/body interactional aerodynamics and, in particular, airframe forces and moments of the X-50 aircraft in hover (both OGE and IGE) and very low speed flight. Rot3DC predictions showed strong sensitivity of fuselage pitching moment to winds, producing high nose-down pitching moments in aft winds. Rot3DC was also able to predict the significant rotor/body aerodynamic cross coupling present in the X-50 aircraft.